1. Field of the Invention
The present invention relates to an image reading apparatus and a light source stability determination method.
2. Description of the Related Art
An image reading apparatus such as a scanner or fax, uses a light source to irradiates light to a manuscript, uses an imaging device to detect amount of light reflected on the manuscript by respective pixels of the imaging device, and generates image signals based on the amount of the light that the respective pixels. In general, the image reading apparatus uses a cold-cathode tube as a light source.
However, a cold-cathode tube has a characteristic that a predetermined period of time, from the time it starts lightning until the time the amount of light it generates becomes stable, is relatively long. In another word, the cold-cathode tube has a characteristic that a period of time that amount of the light obtained from the entire cold-cathode tube becomes even, is relatively long. Accordingly, conventional image reading apparatuses using a cold-cathode tube as a light source stabilize the cold-cathode tube in various ways.
For example, an image reading apparatus disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H3-297266 stabilizes the cold-cathode tube by controlling turning on and turning off of the cold-cathode tube.
An image reading apparatus disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2001-127962 controls a tube current of the cold-cathode tube by monitoring the temperature of the cold-cathode tube, thereby obtains a stable light amount.
Further, an image reading apparatus disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2000-287035 shortens the period of time needed from the time the cold-cathode tube is turned on until the time the generated light amount becomes stable, by flowing a large current through the cold-cathode tube immediately after the tube is turned on.
In addition, image reading apparatuses of these kinds prevents decreasing the precision for scanning by various methods.
For example, an image reading apparatus disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H6-348824 monitors a light amount obtained from a part of a cold-cathode tube, and scans an image when the light amount from this part becomes stable.
Decrement of the quality of the scanned image is attributed not only to unstableness of the light amount obtained from the entire cold-cathode tube, but also to irregularity in the light receiving precision of pixels included in an imaging device. Thus, some image reading includes an imaging device whose pixels having fine light receiving efficiency, such as one disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H11-87675.
Further, some other image reading apparatuses, such as one disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2000-151934, correct quality of the scanned image by shading correction.
The disclosures of the above-indicated documents are incorporated herein by reference.
An image reading apparatus, which performs shading correction, usually scans a reference white plate having a reflection surface whose reflection ratio is almost uniform by using an imaging device, after a cold-cathode tube starts lighting and before image reading is started. The image reading apparatus generates electric signals (white color image signals) having signal levels (white color image densities) corresponding to the amounts of light detected by the respective pixels of the imaging device by scanning, and calculates correction values from the electric signals. Then, the image reading apparatus performs shading correction by using the correction values.
In some case, conventional image reading apparatuses scan the reference white plate to calculate the correction values undesirably when the cold-cathode tube is not stable enough, i.e., when the light amount obtained from the entire cold-cathode tube is uneven. In such a case, the image reading apparatus can not calculate proper correction values, perform shading correction properly neither. Therefore, precision of correcting image is decrease.